WO2011083760A1 - Optical microphone - Google Patents

Abstract

Disclosed is an optical microphone, comprising a light source (1) that outputs a monochromatic light; a two-beam interferometer (3) that divides the monochromatic light that is outputted from the light source (1) into two light beams, conveys the two divided light beams upon two respectively different paths, and thereafter respectively superimposes the two divided light beams and outputs the light thus superimposed; a conveyance medium unit (4), formed from a nanopore body having an acoustic velocity less than the atmospheric acoustic velocity, further comprising an incident unit whereupon audio signals are incident, and that conveys the audio signal that is incident from the incident unit through the nanopore body, traversing one of two paths thereupon; an optical detection apparatus (7) that detects the light from the two-beam interferometer; a first diffuse light removal optical assembly (2a) that is disposed between the two-beam interferometer and the optical detection apparatus, and which removes diffuse light included within the light that is emitted from the two-beam interferometer; and a second diffuse light removal optical assembly that is disposed between the light source (1) and the two-beam interferometer (3), and which removes diffuse light included within the monochromatic light that is emitted from the light source.

Description

Optical microphone

The present invention relates to an optical microphone that detects an acoustic signal using light.

現在 Currently widely used microphones detect acoustic signals using a diaphragm and convert them into electrical signals. Since such a microphone has a mechanical vibration part called a diaphragm, there is a possibility that the characteristics of the diaphragm may be changed by repeatedly using the microphone many times. In addition, if a very strong sound wave is detected with a microphone, the diaphragm may be destroyed. Furthermore, in such a microphone, it is difficult to realize a good broadband characteristic because good acoustic signal detection characteristics cannot be expected at frequencies higher than the minimum resonance frequency of the diaphragm. In addition, if the diaphragm is downsized to achieve wide bandwidth, the detection sensitivity of the acoustic signal decreases.

On the other hand, an optical microphone that detects an acoustic signal using light and converts it into an electric signal has been proposed. The conventional optical microphone disclosed in Patent Document 1 will be described below. As shown in FIG. 16, the conventional optical microphone 141 disclosed in Patent Document 1 includes a receiving mechanism unit 1410 and a laser Doppler vibrometer 148.

The receiving mechanism portion 1410 includes a base portion 143 having a recess and a transparent support plate 145 that is transparent to the laser beam 147. A space formed by the concave portion of the base portion 143 and the transparent support plate 145 is filled with a nanoporous body 142 that is a propagation medium. As the nanoporous body 142, for example, a medium such as a dry gel disclosed in Patent Document 2 having a low sound velocity (acoustic impedance is small) and a small acoustic propagation loss is used. The base portion 143 is provided with an opening 144 for introducing the acoustic signal 149 into the nanoporous body 142. Further, the bottom surface of the concave portion of the base portion 143 is a reflective surface 1411.

The laser Doppler vibrometer 148 irradiates laser light 147 from the outside of the reception mechanism unit 1410, and the laser light 147 passes through the transparent support plate 145 and the nanoporous body 142, is reflected by the reflecting surface 1411, and then nanoporous again. It is arranged to pass through the body 142 and the transparent support plate 145 and return to the laser Doppler vibrometer 148.

The external acoustic signal 149 enters the receiving mechanism 1410 through the opening 144, refracts at the interface between the air and the nanoporous body 142, and enters the nanoporous body 142 with high efficiency. The incident acoustic signal 149 is converted into a dense wave 1412 traveling through the nanoporous body 142. At the spot position on the nanoporous body 142 of the laser beam 147 output from the laser Doppler vibrometer 148, the generated dense wave 1412 is observed as a time variation of density. Since this density change causes a change in refractive index, the refractive index changes with time according to the acoustic signal 149 at the spot position.

As shown in FIG. 16, when the laser beam 147 is incident from the normal direction of the interface between the nanoporous body 142 and the transparent support plate 145, the amount of phase variation that the transmitted laser beam 147 receives from the time variation of the refractive index is It is optically equivalent to the amount of phase fluctuation received when it is assumed that the reflecting surface 1411 oscillates in the normal direction according to the time fluctuation of the refractive index at the spot position. Therefore, the laser beam 147 reflected and returned from the reflecting surface 1411 undergoes a Doppler shift corresponding to the vibration motion of the reflecting surface 1411. The laser Doppler vibrometer 148 obtains a light component for each frequency shift amount by obtaining a Fourier coefficient with respect to the frequency shift amount of the light component subjected to the Doppler shift included in the returning laser light 147 reflected by the reflecting surface 1411. Detect intensity.

The refractive index variation is generally proportional to the sound pressure of the acoustic signal 149, and the Doppler shift amount (frequency change amount) is proportional to the vibration motion speed. The output signal from the laser Doppler vibrometer 148 outputs a signal that is approximately proportional to the time derivative of the acoustic signal 149. An electrical signal corresponding to the acoustic signal can be obtained by performing integration processing or appropriate filter processing on the output signal. Thereby, the optical microphone 141 can be operated as a microphone having desired acoustic characteristics.

JP 2009-85868 A Japanese Patent No. 3633926 JP-A-7-260801

However, in the optical microphone of Patent Document 1, it is necessary to use the laser Doppler vibrometer 148. Laser Doppler vibrometers require a monochromatic light source with good monochromaticity and a highly accurate frequency discrimination circuit. For this reason, it is difficult to reduce the size and cost of the laser Doppler vibrometer 148 itself, and inevitably it is difficult to reduce the size of a conventional optical microphone or to realize a low-cost optical microphone.

An object of the present invention is to solve such problems of the prior art and to provide an optical microphone with high sensitivity, small size and low cost.

The optical microphone of the present invention divides the monochromatic light emitted from the light source and the monochromatic light emitted from the light source into two luminous fluxes, and propagates the divided luminous fluxes through two different paths, respectively. Propagation medium comprising a two-beam interferometer that superimposes the two divided light beams and emits the superimposed light, and a nanoporous body having an incident portion for receiving an acoustic signal and having a sound velocity smaller than that of air A propagation medium section in which an acoustic signal incident from the incident section propagates through the nanoporous body across one of the two paths, and a photodetector that detects light emitted from the two-beam interferometer A first scattered light removing optical system that is provided between the two-beam interferometer and the photodetector and removes scattered light contained in the light emitted from the two-beam interferometer, the light source, Between the two-beam interferometer Vignetting, and a second scattered light removing optical system for removing scattered light included in the monochromatic light emitted from the light source.

In a preferred embodiment, each of the first scattered light removal optical system and the second scattered light removal optical system includes a single mode optical fiber.

In a preferred embodiment, the single mode optical fiber of the first scattered light removal optical system and the single mode optical fiber of the second scattered light removal optical system have the same optical characteristics.

In a preferred embodiment, the optical microphone further includes a sound collecting unit that is provided at an incident portion of the propagation medium unit and focuses the acoustic signal.

In a preferred embodiment, the nanoporous material is a silica dry gel.

In a preferred embodiment, the first scattered light removing optical system further includes at least one focusing lens, and the focal point of the focusing lens is located on the core of the end face of the single mode optical fiber.

In a preferred embodiment, the optical microphone further includes a signal processing unit that receives an output from the photodetector and generates a reception signal corresponding to the acoustic signal from the output by homodyne detection.

In a preferred embodiment, the optical microphone further includes a reference beat signal generation unit and a phase comparison unit, the light source emits two linearly polarized lights having different frequencies, and the reference beat signal generation unit includes the two A reference beat signal is generated based on linearly polarized light, and the phase comparison unit generates a reception signal corresponding to the acoustic signal by heterodyne detection using an output from the photodetector and the reference beat signal. A processing unit is further provided.

In a preferred embodiment, the two-beam interferometer is a Mach-Zehnder interferometer.

In a preferred embodiment, the two-beam interferometer is a Michelson interferometer.

In a preferred embodiment, the two-beam interferometer is a Fizeau interferometer.

According to the optical microphone of the present invention, the time variation of the refractive index of the propagation medium through which the acoustic signal to be detected propagates is determined as the time variation of the optical distance by using a two-beam interferometer which is a kind of distance measuring type interferometer. To detect. Since the two-beam interferometer is relatively small and low cost and has a simple configuration, the configuration of the optical microphone can be reduced. In addition, the scattered light removal optical system is used to remove the scattered light generated by passing through the propagation medium. For this reason, non-interfering components are removed or suppressed from the light detected by the detector, and fluctuations in the optical distance caused by the propagation of acoustic signals can be detected with high sensitivity as interference components in monochromatic light. Become. Therefore, a compact, high-sensitivity and low-cost optical microphone is realized.

1 is a schematic configuration diagram showing a first embodiment of an optical microphone according to the present invention. It is a schematic block diagram which shows the sound collection part in 1st Embodiment. 6 is a graph showing temporal variation of an output signal obtained from the photodetector 7 when it is assumed that there is no scattered light removal optical system 2a, 2b in the first embodiment. It is a schematic block diagram which shows 2nd Embodiment of the optical microphone by this invention. It is a schematic block diagram which shows the 1st Example of the optical microphone by this invention. It is a schematic diagram for demonstrating the function of the scattered light removal optical systems 2a and 2b in a 1st Example. It is a graph which shows the wavelength dependence of the transmittance | permeability of the propagation medium part 4 used in the 1st Example. It is a figure explaining the structure used for the experiment for verifying the removal effect of a scattered light in a 1st Example. 5 is a graph showing a waveform of a received signal detected by a detector in the first embodiment. It is a figure explaining the structure used for the experiment for verifying the removal effect of the scattered light in the case where the scattered light removal optical system is replaced with a conventional aperture stop in the first embodiment. It is a schematic block diagram which shows the 2nd Example of the optical microphone by this invention. It is a schematic block diagram which shows the 3rd Example of the optical microphone by this invention. It is a schematic block diagram which shows the other form of the 3rd Example of the optical microphone by this invention. It is a schematic block diagram which shows the 4th Example of the optical microphone by this invention. It is a schematic block diagram which shows the other form of the 4th Example of the optical microphone by this invention. It is a schematic block diagram which shows the conventional optical microphone. It is a schematic apparatus block diagram which shows the conventional scanning probe microscope. It is a schematic block diagram which shows the conventional optical matched filter.

The inventor of the present application examined a measurement method in place of a laser Doppler vibrometer in a conventional optical microphone. As a result, if the time variation of the refractive index of the propagation medium is not measured as a Doppler shift but is measured as the time variation of the optical distance of the propagation medium, an ordinary distance measuring interferometer having a simple structure is used. I found out that I could do it. Examples of such a normal ranging interferometer include a two-beam interferometer such as a Mach-Zehnder interferometer, a Michelson interferometer, and a Fizeau interferometer. If the detection method of the interferometer is homodyne detection, high monochromaticity is not required for the light source, so the light source can be reduced in size and the entire optical microphone can be further reduced in size.

However, when the present inventor conducted research on an optical microphone using a normal two-beam interferometer, it was found that the light transmitted through the nanoporous material as a propagation medium contains a lot of scattered light. When a lot of scattered light is included in the transmitted light, the scattered light becomes a non-interfering light component in the two-beam interferometer, and the interference light contrast is greatly reduced. For this reason, it has been found that a new problem arises that the measurement sensitivity of the interferometer deteriorates and the sensitivity of the optical microphone decreases.

In order to solve this problem, the inventor of the present application uses an ordinary distance-measuring interferometer as detection means in the optical microphone disclosed in Patent Document 1, and the scattered light contained in the transmitted light from the nanoporous body 142. It was considered to remove.

As conventional means for removing scattered light, for example, scattered light removal means used in the apparatus disclosed in Patent Document 3 is known. FIG. 17 is a schematic configuration diagram of a scanning probe microscope disclosed in Patent Document 3. The scanning probe microscope observes the shape of the sample 153 by optically measuring the amount of bending of the cantilever 152 caused by the interaction (for example, atomic force) between the sample 153 and the cantilever 152. Measurement is performed by irradiating the cantilever 152 with a laser beam from the light source 151 for measuring the amount of bending of the cantilever and capturing the reflected light from the cantilever 152 with the light receiving means 156. The measurement is performed using an optical lever, a homodyne type, or a heterodyne type ranging interferometer.

The light beam captured by the light receiving means 156 includes scattered light from the sample 153 in addition to the reflected light from the cantilever 152. Since the latter does not include the amount of bending of the cantilever 152 and deteriorates the measurement accuracy, the scattered light removing means 155 is provided to reduce it. The scattered light removing means 155 in FIG. 17 is a light shielding plate having a minute opening. Since the scattered light removing unit 155 is set so that only the reflected light from the cantilever 152 passes through the opening, the scattered light does not reach the light receiving unit 156. For this reason, deterioration of measurement accuracy can be prevented.

Also, as another conventional means for removing scattered light, it is conceivable to use an optical matched filter disclosed in Non-Patent Document 1. FIG. 18 is a schematic optical system configuration diagram of the optical matched filter disclosed in Non-Patent Document 1. The lens 162 and the lens 164 have the same focal length f, and are disposed at positions separated from each other by a distance 2f on the optical axis. As shown in FIG. 18, the three surfaces that are separated from the lens 162 and the lens 164 by a distance f are hereinafter referred to as an input surface 161, a Fourier transform surface 163, and an output surface 165.

A plane image of the monochromatic light 166 on the input surface 161 is represented by G. As a specific example of the planar image G, there is an apparatus configuration in which a negative film is placed on the input surface 161 and illuminated with monochromatic light from the back surface. The planar image G is converted into an image F [G] that is two-dimensionally Fourier-transformed on the Fourier transform plane 163 by the action of the lens 162 (F [α] represents a two-dimensional Fourier transform image of α). Then, F [G] on the Fourier transform plane 163 is converted into an image F [F [G]] that has been two-dimensionally Fourier transformed on the output plane 165 by the action of the lens 164. However, since two iterations of Fourier transform are equivalent to not performing Fourier transform (F [F [G]] = G), the original image G is reproduced on the output surface 165.

18 In the optical system of FIG. 18, a filter having an in-plane distribution in phase delay amount and transmittance, that is, a spatial filter, is inserted on the Fourier transform plane 163. It is assumed that the in-plane distribution of the phase delay amount and transmittance of the spatial filter is F [H]. At this time, the image reproduced on the output surface 165 is a convolution of G and H.

Here, the above convolution is defined as follows. The convolution G * H (x, y) of the function G (x, y), H (x, y) defined in two dimensions (x, y) is
(Equation 1) G * H (x, y) = ∫dp dq G (p, q) H (xp, yq)
That is. When the convolution G * H (x, y) has a large value, G and H are called highly correlated.

As described above, when the scattered light S is superimposed on the planar image G on the input surface 161, if F [G] is used as the spatial filter, the image with the scattered light S reduced is reproduced on the output surface 165. The This is because the correlation of the image G with respect to the scattered light S is lower than the autocorrelation of the image G.

The light beam that has passed through the nanoporous material is composed of transmitted light (light beam having the same wavefront and amplitude distribution as the incident light) and scattered light (light beam having a wavefront and amplitude distribution different from the incident light). Therefore, if the transmitted light is G and the spatial filter having the in-plane distribution of the phase delay amount and transmittance of F [G] is inserted into the Fourier transform surface 163, the scattered light S is reduced on the output surface 2005. Light flux.

As described above, it is considered that an optical matched filter can also be applied as a means for removing scattered light from an optical microphone.

However, in the optical microphone using the nanoporous body 142, the scattered light from the nanoporous body 142 exists with high intensity in the vicinity of the transmitted light. For this reason, when the scattered light removing means 155 disclosed in Patent Document 3 is used for removing scattered light from the nanoporous body 142, if the opening of the scattered light removing means 155 is larger than the diameter of the transmitted light beam, high intensity is obtained. Scattered light is mixed into the transmitted light. Further, when the aperture diameter is reduced, the transmitted light intensity itself is weakened and the detection sensitivity of the acoustic signal is lowered.

Therefore, in the optical microphone disclosed in Patent Document 1, instead of the laser Doppler vibrometer 148, a conventional distance-measuring interferometer is used, and the scattered light removing means 155 disclosed in Patent Document 3 is used. It is difficult to effectively remove the scattered light generated by the porous body 142. Therefore, it is impossible to achieve both miniaturization and high sensitivity of the optical microphone.

It is considered that the optical matched filter of Non-Patent Document 1 can effectively remove the scattered light of the nanoporous body 142 in the optical microphone disclosed in Patent Document 1. However, an optical matched filter having the configuration shown in FIG. 18 needs to be added to the optical microphone. In addition, it is necessary to prepare a spatial filter in which the in-plane distribution of the phase delay amount and the transmittance is controlled with high accuracy, and to adjust the position of the spatial filter with respect to the light beam with high accuracy. For this reason, it is difficult to reduce the size and cost of the optical microphone.

Based on such examination results, the inventor of the present application removes the scattered light from the light transmitted through the nanoporous body by using a configuration other than the scattered light removing means disclosed in Patent Document 3 and Non-Patent Document 1. The present inventors have found that a high-sensitivity, small-sized and low-cost optical microphone can be realized by using a normal ranging interferometer. Embodiments of an optical microphone according to the present invention will be described below.

(First embodiment)
A first embodiment of an optical microphone according to the present invention will be described. FIG. 1 schematically shows a configuration of an optical microphone 201 of the present embodiment. As shown in FIG. 1, the optical microphone 201 includes a monochromatic light source 1, a scattered light removal optical system (first scattered light removal optical system) 2a, and a scattered light removal optical system (second scattered light removal optical system). ) 2b, a two-beam interferometer 3, a propagation medium unit 4, a sound collecting unit 5, a photodetector 7, and a signal processing unit 8.

The monochromatic light source 1 emits monochromatic light 10. The monochromatic light 10 is preferably a coherent light beam, that is, a light beam having a momentum and a phase. There is no particular limitation on the wavelength of monochromatic light on the principle of measurement, and light in any wavelength region of the visible region, infrared region, and ultraviolet region may be used. However, from the viewpoint of adjusting the optical system in the optical microphone 201, it is preferable to use the monochromatic light source 1 that emits light in the visible region where the light beam can be easily confirmed.

The monochromatic light source 1 may be a semiconductor laser, a solid laser, or a gas laser. However, it is preferable to use a semiconductor laser in order to reduce the overall outer shape of the optical microphone 201.

The two-beam interferometer 3 divides the monochromatic light 10 emitted from the monochromatic light source 1 into two light beams, propagates the two divided light beams through two different paths, and then splits the two light beams. Are superimposed on each other, and the superimposed light is emitted. For this purpose, the two-beam interferometer 3 includes, for example, light beam splitting elements 17a and 17b and light beam reflecting elements 18a and 18b. Here, “dividing” means broadly dividing the light beam into two light beams, and does not mean only when the light beam is divided into two by a plane parallel to the optical axis.

In the present embodiment, a Mach-Zehnder interferometer will be described as an example of the two-beam interferometer 3. As described in the following embodiments, the two-beam interferometer 3 may be a two-beam interferometer having another optical system configuration such as a Michelson interferometer (including a Twyman Green interferometer) or a Fizeau interferometer. Good. Since such a two-beam interferometer is also relatively small and has a simple configuration, a small and highly sensitive optical microphone can be realized at low cost.

The light beam splitting element 17a is, for example, a semi-transparent mirror. By arranging the light beam splitting element 17a so that the angle formed by the normal line of the reflecting surface of the semi-transparent mirror and the traveling direction of the monochromatic light beam 10 is 135 °, the monochromatic light beam 10 is transmitted by the light beam splitting element 17a. The reflected light is divided into two light fluxes. The intensities of the two divided light beams do not need to be exactly the same. Desirably, the intensities of the two light beams are selected so that a parameter called “contrast” described later is maximized.

As the beam splitting element 17a, a polarization beam splitter may be used instead of the semi-transparent mirror. In this case, the monochromatic light 10 incident on the polarization beam splitter has non-polarized light and a polarization plane having a predetermined angle which is not 0 degrees with respect to the polarization axis (P polarization direction or S polarization direction) of the deflection beam splitter. If the polarization plane direction is a temporally changing light such as linearly polarized light or circularly polarized light, reflected light and transmitted light are generated in the polarization beam splitter. When the polarization beam splitter is used for the light beam splitting element 17a, the divided reflected light and transmitted light have polarization planes orthogonal to each other.

As shown in FIG. 1, the two divided light beams propagate along two different paths. Specifically, the two light beams propagate through the reference path 11 and the probe path 12. Thereafter, in the light beam splitting element 17b, the two light beams are superposed, and the superposed light is emitted from the output end 16. The propagation medium unit 4 is disposed in the probe path 12 which is one of the two paths. As will be described in detail below, the propagation medium portion 4 includes a nanoporous material. When the acoustic signal 6 propagates through the propagation medium part 4 so as to intersect the probe path 12, the acoustic signal 6 is converted into a dense wave that travels through the nanoporous body of the propagation medium part 4. For this reason, the density of the nanoporous material changes, and the time variation of the refractive index of the nanoporous material according to the acoustic signal 6 occurs. Therefore, the optical path length of the probe path 12 changes. As a result, the phase of the light passing through the probe path 12 varies with time according to the acoustic signal 6.

The light splitting element 17b has two paths adjusted so that the light beam passing through the reference path 11 and the light beam passing through the probe path 12 interfere well. Here, “good interference” means that the cross sections of the two light beams are completely overlapped and the wave fronts of the two light beams are coincident. In the case of FIG. 1, such adjustment can be realized by adjusting the angles of the light beam splitting elements 17a and 17b and the light beam reflecting elements 18a and 18b.

The propagation medium part 4 has an incident part on which the acoustic signal 6 is incident, and includes a nanoporous material as described above. The nanoporous material is preferably a solid propagation medium that transmits the monochromatic light 10 and has a sound velocity smaller than that of air. Specifically, the sound velocity of the nanoporous material is smaller than 340 m / sec, which is the sound velocity of air. In general, since a material with a low sound velocity has a relatively low density, reflection at the boundary between an environmental fluid such as air and the nanoporous material is small, and an acoustic wave can be taken into the propagation medium unit 4 with relatively high efficiency.

Specifically, the nanoporous material is a dry gel of an inorganic acid compound or an organic polymer. Preferably, the nanoporous material is a dry gel mainly composed of silica. The nanoporous body has a structure in which silica particles having a diameter of several nm to several tens of nm are randomly and three-dimensionally bonded. Moreover, the sound velocity in the silica dry gel is approximately 50 m / sec or more and 150 m / sec or less, and the sound velocity in the air is small as described above. The density of the silica dry gel is approximately 50 kg / m ³ or more and 200 kg / m ³ or less.

Since the size of the silica particles and the bonding structure cannot be ignored with respect to the wavelength of visible light (wavelength 400 nm to 800 nm), visible light that passes through the nanoporous body is subjected to Rayleigh scattering by the nanoporous body. Therefore, the light flux that has passed through the nanoporous material includes strong scattered light (light flux having a wavefront / amplitude distribution different from that of incident light) in addition to transmitted light (light flux having the same wavefront / amplitude distribution as incident light). The intensity of Rayleigh scattering is highly dependent on the size of the scatterer, and is proportional to the sixth power of the size of the scatterer and inversely proportional to the fourth power of the light wavelength.

Therefore, it is conceivable to suppress Rayleigh scattering in order to reduce the intensity of scattered light generated in the nanoporous material. For that purpose, long wavelength light may be used as the monochromatic light 10, or the size of the silica particles in the nanoporous material and the bonding structure of the silica particles may be reduced. However, in order to sufficiently reduce the intensity of scattered light, it is necessary to use light having a shorter wavelength than the visible region, such as infrared light. In this case, when adjusting the optical system in the optical microphone, the monochromatic light 10 cannot be visually recognized, and it becomes difficult to adjust the optical path. In addition, when the size of the silica particles in the nanoporous material and the bonded structure of the silica particles is reduced, it has good acoustic characteristics, particularly that the sound velocity is slower than that of air, and the silica particles and bonding It is necessary to develop a new nanoporous material with a small structure.

On the other hand, according to the optical microphone 201 of the present embodiment, as will be described in detail below, since it includes the scattered light removal optical system 2a and the scattered light removal optical system 2b, it contains a lot of scattered light. Even if the monochromatic light 10 is used or a nanoporous material that generates high-intensity scattered light is used, the sensitivity is not lowered by the scattered light.

In the optical microphone 201 of the present embodiment, the acoustic signal 6 may be incident on the incident portion of the propagation medium unit 4 using any introduction method and device configuration. However, in order to capture the sound pressure in the nanoporous body of the propagation medium portion 4 as a change in the refractive index of the nanoporous body, the acoustic signal 6 needs to be incident on the nanoporous body so as to propagate as a dense wave (longitudinal wave). There is. Further, it is necessary to reduce the reflection at the interface between the propagation medium section 4 and the environmental fluid around the propagation medium section 4, that is, the air, and to make the acoustic signal 6 enter the propagation medium section 4 with the highest possible efficiency. For this reason, the optical microphone 201 preferably includes the sound collection unit 5.

In general, in a substance whose Poisson's ratio is not exactly zero (including nanoporous materials), longitudinal waves and transverse waves are mixed. For this reason, even if a transverse wave is mainly generated by the input of an acoustic signal, a slight longitudinal wave (coherent wave) is generated and can be measured by the optical microphone of the present invention. However, the detection sensitivity in this case is lower than when directly observing longitudinal waves. Therefore, in order to detect the acoustic signal 6 with high sensitivity, the density fluctuation integrated value in the propagation medium section 4 generated by the acoustic signal 6 is maximized in the region passing through the propagation medium section 4 in the probe path 12. It is desirable to configure the sound collecting unit 5 and the propagation medium unit 4 as described above.

FIG. 2 shows an example of the configuration of the sound collection unit 5 that can be used in the optical microphone 201 of the present embodiment. As shown in FIG. 2, the sound collection unit 5 includes a horn 1801, an acoustic waveguide 1802, and a propagation medium unit 4. The acoustic signal 6 that has propagated in the air (the dense wave in the air) is converted into a plane wave (the dense wave in the air) that travels through the acoustic waveguide 1802 by the horn 1801 and propagates toward the acoustic lens 1803. The acoustic lens 1803 has a refractive surface 1804 and is composed of the above-described nanoporous material. Preferably, a non-reflective terminal 1806 for suppressing reverberation is provided at the end of the propagation medium section 4. The refracting surface 1804 is configured such that the acoustic signal 6 incident on the propagation medium unit 4 is refracted toward the focal point 1805. That is, the refractive surface 1804 provided in the propagation medium unit 4 functions as an acoustic lens.

The acoustic signal 6 propagated through the acoustic waveguide 1802 is refracted at the refracting surface 1804 and converted into a spherical wave propagating through the propagation medium section 4. At this time, generation of a transverse wave on the refracting surface 1804 is suppressed, and the wave is converted into a longitudinal wave with high efficiency. For this reason, the detection sensitivity of the acoustic signal 6 is preferably increased.

The acoustic signal 6 incident on the propagation medium unit 4 propagates through the propagation medium unit 4 so as to be focused on the focal point 1805 by refraction at the refracting surface 1804. At the focal point 1805, the acoustic signals 6 are all concentrated in the same phase, and a high sound pressure concentration state is obtained. Therefore, the density fluctuation amplitude of the propagation medium unit 4 at the focal point 1805 becomes large, and the amplitude of the refractive index fluctuation becomes very large. Therefore, if the optical path is arranged so that the monochromatic light 10a propagating along the probe path 12 includes a region in the vicinity of the focal point 1805, the monochromatic light 10 that has passed through the propagation medium section 4 is transmitted through the propagation medium section 4. Crosses signal 6 and undergoes large phase variations in response to refractive index variations. For this reason, highly sensitive acoustic signal reception becomes possible.

However, the optical path of the monochromatic light 10 does not have to be as large as possible so as to include the focal point 1805. If the state of image formation near the focal point 1805 is viewed in detail from the viewpoint of wave optics, the distribution of sound pressure is the Airy disk in which the sound pressure converges in phase, and a plurality of diffraction rings that surround it (this Sound pressure concentration also exists in the area). The sound pressure is also concentrated in the diffraction ring, but the acoustic signals are concentrated at a phase different from that of the Airy disk. For this reason, if the optical path of the laser beam 147 includes the Airy disc region and the diffraction ring region, the acoustic signal is weakened due to a phase shift. Therefore, the optical path of the laser beam 147 is preferably arranged so as to include only the Airy disk.

Next, functions of the scattered light removing optical systems 2a and 2b will be described. From the monochromatic light 10 emitted from the monochromatic light source 1, the scattered light is removed by the scattered light removing optical system 2b, and only the plane wave is filtered. As the parameter called contrast is increased, the measurement sensitivity of the two-beam interferometer 3 is improved. In general, in a two-beam interferometer, one light beam is decomposed into two light beams having the same light intensity, and the two light beams are superposed so that each light beam passes through different paths and becomes one light beam again. Depending on the optical path length difference between the two paths, the light intensity of one superimposed light beam varies. Here, the optical path length difference is a value obtained by multiplying the physical length of the path by the refractive index of the medium. In this light intensity fluctuation, the magnitude of the light intensity fluctuation amplitude (= (light intensity maximum value + light intensity minimum value) / 2) with respect to the unchanged light intensity (= (light intensity maximum value−light intensity minimum value) / 2). Is called contrast. When the two light beams interfere completely, the light intensity minimum value = 0 and the contrast = 1, but when the two light beams do not interfere at all, the light intensity minimum value = the light intensity maximum value and the contrast = 0. As described above, when the contrast = 1, the measurement sensitivity of the two-beam interferometer is maximized.

Therefore, in order to achieve contrast = 1 in the two-beam interferometer 3 of the optical microphone 201, it is necessary that the two light beams divided into equal intensities interfere completely. Interference is a phenomenon that occurs only between photons whose frequency (energy) and propagation direction (momentum) are completely identical. Therefore, in order to realize the contrast = 1, the two light beams must be photons having exactly the same frequency and propagation direction. If the state of such a photon group is viewed in terms of wave optics, in order to achieve contrast = 1, it is necessary that the two light beams are plane waves having the same phase in a situation where two light beams are superimposed on one light beam. More specifically, referring to the apparatus configuration of FIG. 1, the monochromatic light 10 is a plane wave immediately before entering the light beam splitting element 17a, and the light flux that has passed through each of the reference path 11 and the probe path 12 is the output end 16. In the case of a plane wave, the measurement sensitivity of the two-beam interferometer 3 is maximized.

However, in general, the monochromatic light 10 emitted from the monochromatic light source 1 is not a plane wave. Even if the monochromatic light 10 emitted from the monochromatic light source 1 is a plane wave, the monochromatic light 10 is no longer a plane wave when the monochromatic light 10 passes through the propagation medium portion 4 which is a light scattering medium. For this reason, the contrast is lowered, and the measurement sensitivity of the two-beam interferometer 3 is deteriorated from an ideal value. The optical microphone 201 includes scattered light removal optical systems 2a and 2b in order to minimize this deterioration.

As shown in FIG. 1, the scattered light removal optical system 2b removes the scattered light contained in the monochromatic light, and improves the flatness of the wavefront just before entering the light beam splitting element 17a. Further, the scattered light removal optical system 2 a removes the scattered light generated by the nanoporous body of the propagation medium portion 4 and improves the flatness of the wavefront of the light flux at the output end 16. For example, when a semiconductor laser is used as the monochromatic light source 1, since the emitted light from the semiconductor laser is not a plane wave, the scattered light removing optical system 2 b is necessary to improve the measurement sensitivity of the two-beam interferometer 3.

The scattered light removing optical systems 2b and 2b remove scattered light components whose propagation direction is not parallel to the optical axis direction, which is included in the monochromatic light 10. Various optical elements and optical systems having such functions can be used as the scattered light removing optical systems 2a and 2b. For example, since a single mode optical fiber can only propagate light of a single mode, the scattered light removing optical systems 2b and 2b that remove scattered light can be suitably used. In addition, a polarization-maintaining optical fiber can also be used.

From the viewpoint of measurement sensitivity of the two-beam interferometer 3, the wavefront deviation from a complete plane in the cross section of the light beam emitted from the scattered light removal optical system 2 b is preferably less than one wavelength in terms of the wavelength of the monochromatic light 10.

The scattered light removal optical system 2a is also provided with the same optical system structure as the scattering optical system 2b, and is most preferably a single mode optical fiber. From the viewpoint of measurement sensitivity of the two-beam interferometer 3, the wavefront deviation from a complete plane in the section of the light beam emitted from the scattered light removal optical system 2 a may be less than one wavelength in terms of the wavelength of the monochromatic light 10. preferable.

With the above-described configuration, interference light having high contrast is emitted from the two-beam interferometer 3. For this reason, it becomes possible to detect the acoustic signal 6 with high sensitivity. The light emitted from the two-beam interferometer 3 is converted into an electrical signal by the photodetector 7, and homodyne detection is performed using the signal processing unit 8, whereby the acoustic signal 6 can be detected as the reception signal 9.

FIG. 3 is a graph showing the time variation of the output signal from the photodetector 7 when it is assumed that there is no scattered light removal optical system 2a and no scattered light removal optical system 2b. The homodyne detection is a detection method that captures the time variation of the optical path length difference between the reference path 11 and the probe path 12 caused by the refractive index variation in the propagation medium unit 4 as the time variation of the interference light intensity. Therefore, when the scattered light removal optical system 2a and the scattered light removal optical system 2b are not provided, the output signal from the photodetector 7 includes a non-interference signal that does not include an acoustic signal in addition to an interference component that includes the acoustic signal.

The signal processing unit 8 includes a DC cut filter 13 and an amplifier 14, and after the DC component is removed by the DC cut filter 13 from the output signal from the photodetector 7, the signal is amplified by the amplifier 14 and the acoustic signal 6 is received. 9 is generated.

For the purpose of noise reduction, the monochromatic light source 1 is a light source whose output is stabilized. Therefore, the output light intensity of the monochromatic light source 1 is constant over time, and the maximum signal value in FIG. 3 is substantially constant. Therefore, if the gain of the amplifier 14 is constant over time, the amplitude of the received signal 9 is proportional to the ratio of the interference component to the maximum signal value in FIG. Therefore, without the scattered light removal optical system 2a and the scattered light removal optical system 2b, the non-interference component increases, and the amplitude of the received signal 9 obtained decreases. That is, the reception sensitivity of the acoustic signal 6 is deteriorated.

On the other hand, by using the scattered light removing optical system 2a and the scattered light removing optical system 2b, the scattered light component, that is, the non-interference component is excluded from the light beam incident on the photodetector 7, thereby increasing the amplitude. The received signal 9 is obtained, and the amplitude ratio (S / N ratio) of the acoustic signal 9 to noise is also increased. Therefore, the optical microphone 201 with high reception sensitivity is realized.

As described above, according to the optical microphone of the present embodiment, the time variation of the refractive index of the nanoporous material through which the acoustic signal to be detected is propagated using a two-beam interferometer that is a kind of distance measuring interferometer, It is detected as a change in optical distance. Since the two-beam interferometer is relatively small and has a simple configuration, the configuration of the optical microphone can be reduced. Further, the scattered light removal optical system is used to remove the scattered light generated when the monochromatic light used for measurement passes through the propagation medium. For this reason, non-interfering components are removed or suppressed from the light detected by the detector, and fluctuations in the optical distance caused by the propagation of acoustic signals can be detected with high sensitivity as interference components in monochromatic light. Become. Therefore, a small and highly sensitive optical microphone is realized.

(Second Embodiment)
A second embodiment of the optical microphone according to the present invention will be described. FIG. 4 schematically shows the configuration of the optical microphone 202 of the present embodiment. In FIG. 4, the same reference numerals are assigned to the same or corresponding components as those in the first embodiment. The optical microphone 202 includes a two-frequency light source 1703, a scattered light removing optical system 2a, a scattered light removing optical system 2b, a two-beam interferometer 3, a propagation medium unit 4, a photodetector 7, and a non-polarizing beam splitter. 1712, a phase comparison unit 1705, a polarizing plate 1706, and a reference beat signal generation unit 1711.

The optical microphone 202 is different from the first embodiment in that the acoustic signal 6 is detected by heterodyne detection. Therefore, in the optical microphone 202, the monochromatic light source 1 of the optical microphone 201 of the first embodiment is changed to a dual-frequency light source 1703, and further includes a reference beat signal generation unit 1711.

The dual-frequency light source 1703 generates dual-frequency light 1704 composed of two linearly polarized light having mutually orthogonal polarization planes. In FIG. 4, the angle of the dual frequency light source 1703 with respect to the optical axis is such that one polarization plane of linearly polarized light is parallel to the plane of FIG. 4 and the other plane of polarization is perpendicular to the plane of FIG. Has been adjusted.

The two linearly polarized lights are monochromatic lights having different frequencies, and here, the frequency difference between the two linearly polarized lights is expressed as Δf. As the dual-frequency light source 1703 having such characteristics, for example, a dual-frequency Zeeman laser can be used. In addition, the linearly polarized monochromatic light is divided into two parts, and both or one of the linearly polarized monochromatic lights is changed in frequency so that the frequency difference between the two linearly polarized monochromatic lights is Δf using an acoustooptic device. Then, a light source optical system that superimposes linearly polarized monochromatic light so that the polarization planes of the two linearly polarized monochromatic light are orthogonal to each other may be used.

The two-frequency light 1704 emitted from the two-frequency light source 1703 is divided by the non-polarizing beam splitter, a part thereof is guided to the reference beat signal generation unit 1711, and the rest is guided to the scattered light removing optical system 2b. The reference beat signal generation unit 1711 includes a polarizing plate 1710 and a photodetector 1707. The polarizing plate 1710 has a polarization axis that forms an angle of 45 ° with respect to the paper surface of FIG. 4, and the two-frequency light 1704 that has entered the reference beat signal generation unit 1711 passes through the polarizing plate 1710, so that 2 The two linearly polarized monochromatic lights in the frequency light 1704 have a common plane of polarization and interfere with each other. The interference light generated in this way becomes beat light whose intensity changes sinusoidally at the frequency Δf. This beat light is converted into an electric signal by the photodetector 1707, and a reference beat signal 1708 is generated.

The two-frequency light 1704 transmitted through the non-polarizing beam splitter 1712 is split by the polarizing beam splitter 1701 in accordance with the polarization plane direction after passing through the scattered light removing optical system 2b. The linearly polarized monochromatic light (P-polarized light) having a polarization plane parallel to the paper surface of FIG. 4 is directed toward the propagation medium section 4 and has a polarization plane perpendicular to the paper surface of FIG. Wave monochromatic light (S-polarized light) is reflected in the direction of the light reflecting element 18b. As described in the first embodiment, when the P-polarized light is transmitted through the propagation medium unit 4 in which the acoustic signal 6 is propagating, the phase change corresponding to the refractive index change generated in the propagation medium unit 4 is generated. receive.

S-polarized light and P-polarized light are superimposed by a non-polarizing beam splitter 1702. However, since the planes of polarization are orthogonal to each other, no interference light is generated at this point. For this reason, similarly to the reference beat signal generation unit 1711, interference is caused by passing through a polarizing plate 1706 having a polarization axis that forms an angle of 45 ° with respect to the paper surface of FIG. Produce light. This interference light also becomes beat light whose intensity changes sinusoidally at the frequency Δf, but is different from the reference beat signal 1708 in that it has a phase change corresponding to the refractive index change generated in the propagation medium section 4.

As in the first embodiment, the scattered light is removed from the beat light by the scattered light removing optical system 2 a and detected by the photodetector 7. As a result, a probe beat signal 1709 is generated.

The phase comparator 1705 heterodyne-detects the probe beat signal 1709 using the reference beat signal 1708. Specifically, the phase change of the probe beat signal 1709 is extracted with the phase of the reference beat signal 1708 as a reference, and the reception signal 9 is output. This phase change is a phase change corresponding to the refractive index change caused by the acoustic signal 6. Therefore, the acoustic signal 6 is detected as the received signal 9.

Ideally, the reference beat signal 1708 and the probe beat signal 1709 have a signal waveform constituted by the sum of a DC signal that does not change with time and a sine signal having a frequency Δf. Heterodyne detection has a function of extracting a phase difference between two signals regardless of the magnitude of the DC signal and the amplitude of the sine signal of the two signals (reference beat signal 1708 and probe beat signal 1709) to be phase-compared. . Therefore, the optical microphone 202 based on the heterodyne detection system of this embodiment has an advantage that it always gives a detection sensitivity to a constant acoustic signal 6 regardless of fluctuations in the magnitude of the DC signal and the amplitude of the sine signal.

However, since the output fluctuation of the two-frequency light source 1703 and the interference alignment fluctuation of the two-beam interferometer 3 due to external vibration actually occur, the magnitude of the DC signal and the amplitude of the sine signal can fluctuate with time. . When this temporal variation is included in the temporal variation of the sine wave having the frequency Δf, the reference beat signal 1708 and the probe beat signal 1709 include other than the sine wave having the frequency Δf from the beat light. A sine wave will be included. As a result, the other sine wave causes a phase change of the sine wave of frequency Δf. Since this phase change is a time variation that does not depend on the acoustic signal 6, it appears as noise in the received signal 9.

Therefore, in order to reduce such noise, the DC signal component included in the reference beat signal 1708 and the probe beat signal 1709 may be reduced. As described in the first embodiment, the DC signals included in the reference beat signal 1708 and the probe beat signal 1709 mainly depend on the scattered light intensity. For this reason, by removing scattered light with high efficiency by the scattered light removing optical systems 2a and 2b, noise included in the received signal 9 can be reduced, and an optical microphone having a high S / N ratio can be realized. it can.

As described above, in the optical microphone of this embodiment, as in the first embodiment, the acoustic signal to be detected is a two-beam interference that is relatively small in size and has a simple configuration because the acoustic signal to be detected changes the refractive index of the nanoporous material over time. It can be detected as a change in optical distance using a meter. Further, by using the scattered light removing optical system, it is possible to suppress the influence of noise and detect an acoustic signal with high sensitivity. Therefore, a small and highly sensitive optical microphone is realized.

Hereinafter, a first embodiment of the optical microphone according to the present invention will be described.

FIG. 5 shows the configuration of an optical microphone 211 that is a specific embodiment of the present invention. In FIG. 5, the same components as those in the first embodiment are denoted by the same reference numerals.

As shown in FIG. 5, the optical microphone 211 includes a Mach-Zehnder interferometer 36 constituted by two reflecting mirrors 35a and 35b and beam splitters 34a and 34b as a two-beam interferometer. The scattered light removing optical system 2a includes an optical fiber 33a and focusing lenses 31a and 32a provided at both ends thereof. Similarly, the scattered light removal optical system 2b includes an optical fiber 33b and focusing lenses 31b and 32b provided at both ends thereof.

Each of the optical fibers 33a and 33b is a single mode optical fiber that propagates the monochromatic light 10 having a predetermined wavelength in a single mode, and the scattered light removing optical systems 2a and 2b are mirror-symmetrical optical systems with respect to the Mach-Zehnder interferometer 36. Is configured. Specifically, the optical fiber 33a and the optical fiber 33b have the same optical characteristics. Here, the optical characteristics of the optical fiber 33a and the optical fiber 33b are the same, for example, the cross section perpendicular to the optical axis has the same geometric shape and the same refractive index distribution, and the focusing lens. This means that the end faces facing 31a and the converging lens 31b have the same end face shape. Similarly, the focusing lens 31a and the focusing lens 31b have the same optical characteristics. Here, the optical characteristics of the focusing lens 31a and the focusing lens 31b being the same means, for example, that the intensity and phase distribution of the light formed on the image plane are the same for the same incident light. However, the focusing lens 32a and the focusing lens 32b may not have the same optical characteristics. In this case, it is possible to realize an optical microphone that is excellent in terms of the effect of removing scattered light generated in the propagation medium unit 4 to improve reception sensitivity, and in terms of the ease of assembling and adjusting the optical system.

The focusing lens 32b is preferably one that efficiently couples the monochromatic light 10 to the optical fiber 33b. As such a lens, for example, a short focus lens having excellent imaging characteristics at the wavelength of the monochromatic light 10 such as a GRIN lens (refractive index distribution lens) or a microscope objective lens is preferably used. In order to realize a small and highly sensitive optical microphone, the numerical aperture of the converging lens 32b can be used to irradiate the propagation medium section 4 with a light beam with the highest possible intensity while using the small output monochromatic light source 1. NA is preferably equal to or less than the numerical aperture NA of the optical fiber 33b. When the monochromatic light 10 enters the optical fiber 33b and propagates through the optical fiber 22, the scattered light included in the monochromatic light cannot be coupled to the optical fiber 33b and cannot propagate through the optical fiber 33b. For this reason, the scattered light in the monochromatic light 10 is removed by propagating through the optical fiber 33b.

The focusing lenses 31a and 31b are preferably those capable of satisfactorily coupling a parallel light beam (Gaussian light beam) having a good wavefront to the optical fibers 33a and 33b, respectively. As such a lens, a focusing lens such as a dedicated GRIN lens or an aspherical lens optimized for the wavelength used is suitable. As long as it is connected to the focusing lens via an optical fiber connector, an optical element that can accurately match the end face of the optical fiber and the focal position of the lens is commercially available, and such an optical element is preferably used. Can do.

The reason why the scattered light removing optical systems 2a and 2b configured as described above can extremely effectively suppress the scattered light generated in the propagation medium unit 4 will be described with reference to FIG. The focal point of the focusing lens 31b coincides with the core of the optical fiber 33b. For this reason, the monochromatic light 601 in the optical fiber is converted into coherent light φ ₀ which is a parallel light beam by the converging lens 31b. The structure of the optical fiber 33a and the focusing lens 31a, since the optical fiber 33b is the same as that of the focusing lens 31b, coherent light phi ₀ through focusing lens 31a is collected on the core of the optical fiber 33a end face.

Monochromatic light propagating through a single mode optical fiber has a fixed intensity / phase distribution (more precisely, an electromagnetic field distribution). The intensity / phase distribution is uniquely determined from the configuration of the single mode optical fiber (refractive index distribution) and the monochromatic light wavelength. On the contrary, monochromatic light having no fixed intensity / phase distribution cannot propagate through the single mode optical fiber. When the nanoporous body 4 is not present, the intensity / phase distribution image of the monochromatic light 601 in the optical fiber formed on the end face of the optical fiber 33b is projected onto the end face of the optical fiber 33a by the focusing lenses 31a and 31b. In the configuration of FIG. 6, this image is equal to the intensity / phase distribution of the monochromatic light 601 in the optical fiber formed on the end face of the optical fiber 33b. In addition, this image exactly matches the intensity / phase distribution formed on the end face of the optical fiber 33a by monochromatic light that can propagate through the optical fiber 33a. For this reason, the monochromatic light 601 propagating through the optical fiber 33b becomes coherent light φ ₀ and is introduced into the optical fiber 33a with an efficiency close to 100%.

Next, a case where the nanoporous body 4 is inserted will be described with reference to FIG. As described above, the nanoporous body 4 generates scattered light Σφ _i (the sum is taken for i of i ≠ 0). Scattered light is light having an intensity / phase distribution different from incident light. Converging lens 31a is an image of the transmitted light and nanoporous 4 scattered light Sigma] [phi _i generated by from nanoporous 4 is formed on the end face of the optical fiber 33a. The image of the scattered light Σφ _i is not introduced into the optical fiber 33a because the monochromatic light that can propagate through the optical fiber 33a does not match the intensity / phase distribution formed on the end face of the optical fiber 33a. However, since the transmitted light is equal to the coherent light φ ₀ , it is introduced into the optical fiber 33a with high efficiency. Therefore, the monochromatic light 602 in the optical fiber does not include the scattered light generated by the nanoporous body 4 and includes only the coherent light φ ₀ . The above is the reason why the scattered light removing optical systems 2a and 2b can extremely effectively suppress the scattered light generated in the propagation medium section 4.

Although not shown in FIG. 6, when introducing the monochromatic light 10 into the scattered light removing optical system 2b, the monochromatic light 10 is transmitted from the left end face toward the optical fiber 33b via an additional optical system such as a condensing optical system. It is preferable to make it enter. At this time, the electromagnetic field distribution of the monochromatic light 10 generated on the left end face of the optical fiber 33b by the additional optical system light does not exactly match the electromagnetic field distribution of the light that can propagate through the optical fiber 33b. For this reason, only the component of the electromagnetic field distribution of light that can propagate through the optical fiber 33b is passively selected, and the other components are scattered and reflected by the left end face of the optical fiber 33b. Thus, the scattered light removal optical system 2b functions as an optical system that “shapes” the electromagnetic field distribution of the monochromatic light 10 into a desired electromagnetic field distribution (wavefront and intensity distribution). With this function of the scattered light removing optical system 2b, the electromagnetic field distribution of light incident on the propagation medium section 4 is purified, and the filtering performance as a mode filter is enhanced.

Next, the results of experiments on the specific scattered light removal effect by the scattered light removal optical systems 2a and 2b having the configuration shown in FIG. 6 will be described. The optical microphone 211 shown in FIG. 5 was produced, and the configuration shown in FIG. 6 was used as the scattered light removing optical systems 2a and 2b. A He—Ne laser (wavelength: about 633 nm) was used as the monochromatic light source 1.

FIG. 7 shows the wavelength dependence of the transmittance of the propagation medium unit 4 used in the optical microphone 211. The transmittance was measured with an integrating sphere. As can be seen from FIG. 7, the decrease in transmittance caused by Rayleigh scattering begins to become noticeable at 800 nm. Therefore, in the visible light region, the intensity of scattered light from the nanoporous material is large, which is a typical light scattering medium.

The thickness of the propagation medium part 4 used in the experiment is 5 mm. In FIG. 7, the result of having measured the transmittance | permeability with respect to the sample produced on three types of different conditions is shown. From FIG. 7, it can be seen that the propagation medium unit 4 used in this experiment has a translucency of 80% to 90% with respect to monochromatic light having a wavelength of about 633 nm emitted from the monochromatic light source 1.

From this result, it seems that monochromatic light having a wavelength of about 633 nm is well transmitted through the propagation medium part 4 and does not generate much scattered light. However, actually, from the results obtained as a secondary result from the experiments described below, it has been found that the coherent light contained in the light flux that has passed through the propagation medium section 4 is about 8% in intensity ratio. For this reason, it is expected that the monochromatic light 10 transmitted through the propagation medium portion 4 includes high-intensity scattered light in a small angle region with respect to the emission direction of the coherent light beam.

Moreover, it can be seen from FIG. 7 that if long wavelength monochromatic light is used, the generation of scattered light in the nanoporous material is small. For this reason, if an optical microphone is realized using monochromatic light in the infrared region, it is considered that the influence of scattered light can be reduced without using a scattered light removal optical system.

However, when an experiment was conducted using a 1.3 μm semiconductor laser based on such examination, the monochromatic light was infrared light, so the laser light could not be seen and interference alignment was impossible. . In addition, I tried using a card type laser light detection target (made by Thorlabs, model number VRC2) that converts infrared light into visible light, but it was difficult to observe detailed interference fringes in the laser spot, so good interference Alignment could not be realized.

Next, in order to observe how much scattered light is attenuated by the scattered light removing optical systems 2a and 2b using the configuration shown in FIG. 9, an experiment was performed according to the following procedure.
(1) Only the light shielding A is arranged in the Mach-Zehnder interferometer 36 to block the optical path on one side, and the light intensity Va is measured by the photodetector 7.
(2) Only the light shield B is arranged, the optical path on the opposite side to (1) is blocked, and the light intensity Vb is measured by the photodetector 7.
(3) The light shielding A and B are removed, and the amplitude Vex of the interference component is measured by the photodetector 7. The measurement of the amplitude Vex can be performed, for example, by shaking the reflecting mirror 35a with a finger to read the maximum / minimum values of the output value from the light detector 7 and obtaining 1/2 of the difference.
(4) If the light Va and Vb completely interfere, the amplitude of the interference light is ideally Vth = (Va × Vb) ^1/2 . Therefore, the scattered light removal effect by the optical system shown in FIG. 8 can be evaluated by comparing the theoretical value Vth of the interference light amplitude and the actual measurement value Vex.

FIG. 9 shows an example of the waveform of the received signal 9 measured by the above procedure. As a result of measurement according to the procedure shown in (1) to (4), Vex≈ (0.90 ± 0.11) × Vth. From this, it was confirmed that the scattered light generated in the propagation medium part 4 could be removed almost completely within the range of the accuracy of the experiment.

Generally, when constructing a laser optical system, an aperture stop is often arranged in the optical path for the purpose of removing scattered light and stray light. In the experimental system shown in FIG. 8, in order to determine whether such a conventional method can be applied, as shown in FIG. 10, the scattered light removing optical system 2a is replaced with an aperture stop 81, and an acoustic signal 6 is input. At this time, it was examined whether or not a signal corresponding to the signal was output from the photodetector 7. An acoustic signal 6 having an intensity that can be detected with the configuration shown in FIG. 8 was input and detection was attempted by the photodetector 7, but the signal output could not be confirmed. Therefore, it was found that the conventional aperture stop 81 alone does not function as the scattered light removal optical system 2a. This is probably because the original interference light intensity also decreases with the reduction of the aperture diameter, and the S / N deteriorates, making it difficult to filter the desired signal.

From these results, it was found that the scattered light removal optical systems 2a and 2b function suitably in order to remove scattered light in the propagation medium section 4 having the optical characteristics shown in FIG.

Hereinafter, a second embodiment of the optical microphone according to the present invention will be described. In the first embodiment, the Mach-Zehnder interferometer 36, which is one type of the two-beam interferometer 3, is configured using bulk optical elements. However, in order to improve the characteristics of the two-beam interferometer, a large number of bulk optical elements are used. There is a problem that optical adjustment must be made precisely. In the present embodiment, the Mach-Zehnder interferometer 36 is configured using an optical fiber, thereby reducing the number of bulk optical elements and eliminating the optical adjustment required for each of the bulk optical elements, thereby solving this problem. .

As shown in FIG. 1, the optical microphone 212 is different from the optical microphone 211 of the first embodiment in that it includes an optical fiber type Mach-Zehnder interferometer 42 having a path constituted by optical fibers. The optical fiber type Mach-Zehnder interferometer 42 includes a 1-input 2-output coupler 41b, a 2-input 1-output coupler 41a, an optical fiber for the reference path 11, and an optical fiber for the probe path 12.

In the optical microphone 212, one end of the optical fiber 33b which is the scattered light removing optical system 2b is connected to the input of the coupler 41b. One end of the optical fiber for the reference path 11 and one end of the optical fiber for the probe path 12 are connected to the output of the coupler 41b. The other end of the optical fiber for the reference path 11 and the other end of the optical fiber for the probe path 12 are connected to the input of the coupler 41a. One end of the optical fiber 33a which is the scattered light removing optical system 2a is connected to the output of the coupler 41a. The optical fiber constituting the probe path 12 is cut halfway, and the propagation medium portion 4 is inserted. With the above configuration, it is possible to construct an optical fiber type Mach-Zehnder interferometer 42 that does not use any bulk optical element as a two-beam interferometer.

As described in the above embodiment, the sensitivity of the optical microphone 212 is determined by the contrast of the interference light output from the optical fiber type Mach-Zehnder interferometer 42 (ratio of the interference light amplitude to the total output light amplitude). This contrast depends on the adjustment of the optical element in addition to the degree of removal of scattered light from the propagation medium unit 4. Specifically, in order to realize good reception sensitivity with respect to the acoustic signal 6, not only complete superimposition of the light fluxes of the reference path 11 and the probe path 12, but also the coincidence of the wavefronts of both light fluxes is used. Optical adjustment of the Zender interferometer 42 is required. In the optical microphone 211 shown in FIG. 5, adjustment of four bulk optical elements is necessary for such optical adjustment. However, according to the present embodiment, all of these adjustments are unnecessary and are completely completed. Interference adjustment is always realized.

According to the optical microphone 212, the optical system in the optical microphone is routed by an optical path as much as possible, thereby reducing the cost of adjusting the optical system and generating interference light with higher contrast. . Therefore, an optical microphone having high acoustic signal detection sensitivity can be realized.

Hereinafter, a third embodiment of the optical microphone according to the present invention will be described. In principle, any two-beam interferometer may be used in the optical microphone of the present invention. In this embodiment, a Michelson interferometer is used instead of the Mach-Zehnder interferometer.

12, the optical microphone 213 is different from the optical microphone 211 shown in FIG. 5 in that a Michelson interferometer 91 is provided instead of the Mach-Zehnder interferometer 36.

As shown in FIG. 12, the Michelson interferometer 91 includes reflecting mirrors 35a and 35b and a beam splitter 34a. The monochromatic light 10 emitted from the focusing lens 31b coupled to the optical fiber 33b is divided into monochromatic light propagating along the reference path 11 and monochromatic light propagating along the probe path 12 by the beam splitter 34a. The monochromatic light propagating along the reference path 11 is reflected by the reflected light 35a and enters the beam splitter 34a again. Further, the monochromatic light propagating through the probe path 12 is transmitted through the propagation medium section 4, is reflected by the reflecting mirror 35b, is transmitted through the propagation medium section 4 again, and enters the beam splitter 34a. The two monochromatic lights incident on the beam splitter 34a are superimposed and enter the optical fiber 33a via the focusing lens 31a.

As described above, the Michelson interferometer 91 includes the three bulk optical elements of the reflecting mirrors 35a and 35b and the beam splitter 34a. For this reason, it is necessary to adjust the optical position of these optical elements. However, the use of the reflecting mirrors 35a and 35b has the advantage that the optical path can be turned back, and the Michelson interferometer 91 can be made compact. Further, since the monochromatic light 10 travels back and forth through the propagation medium section 4, the detection sensitivity of refractive index fluctuation is doubled. In order to improve the acoustic detection sensitivity without changing the configuration of the sound collection unit 5, the propagation medium unit 4 that is thick in the optical path direction may be used. However, due to the increase in size of the propagation medium section 4, there is a case where the broadband property is impaired due to the acoustic propagation characteristics of the propagation medium section 4, particularly the influence of the self-resonance mode. On the other hand, if the Michelson interferometer 91 is used, the detection sensitivity can be increased without changing the size of the propagation medium section 4. Therefore, it is possible to achieve both improvement of the broadband property and the sound detection sensitivity.

As described above, by using the Michelson interferometer 91 as the two-beam interferometer, it is possible to realize an optical microphone having a small size and high detection sensitivity while having a wide bandwidth.

Further, as shown in FIG. 13, the interference light obtained from the Michelson interferometer 91 may be returned to the optical fiber 33a. An optical microphone 213 ′ shown in FIG. 13 includes a monochromatic light source 1, a beam splitter 34b, a scattered light removing optical system 2b including an optical fiber 33a, a Michelson interferometer 91, a photodetector 7, and a signal processing unit 8. Is provided.

The monochromatic light 10 emitted from the monochromatic light source 1 passes through the beam splitter and enters the scattered light removing optical system 2b. The monochromatic light 10 from which the scattered light component has been removed in the scattered light removing optical system 2b is incident on the Michelson interferometer 91 as described above. The light emitted from the Michelson interferometer 91 is again incident on the scattered light removing optical system 2b in the opposite direction, and the monochromatic light emitted from the scattered light removing optical system 2b is again incident on the beam splitter 34b. The acoustic signal 6 can be detected by reflecting the monochromatic light 10 toward the photodetector 7 in the beam splitter 34b.

According to such a configuration, since the input / output end of light to the Michelson interferometer 91 is one point, the optical microphone can be configured in a small size. Further, the scattered light removal optical systems 2a and 2b can be shared without impairing the scattered light removal effect, so that the optical microphone can be further reduced in size, and the manufacturing cost can be reduced.

Hereinafter, a third embodiment of the optical microphone according to the present invention will be described. FIG. 14 shows the configuration of the optical microphone 214 of this embodiment. The optical microphone 214 differs from the optical microphone 213 ′ of the third embodiment in that it includes the Fizeau interferometer 101.

Although the Mach-Zehnder interferometer and the Michelson interferometer have advantages unique to their respective configurations, the probe path 12 and the reference path 11 are spatially different. In particular, in the Mach-Zehnder interferometer, the spatially different portions in the two paths are long. For this reason, due to mechanical instability, the optical path difference between the two paths may fluctuate and may be observed as an unnecessary interference signal. Even if the mechanical parts and optical elements are robustly constructed, the air fluctuation, temperature distribution, and temperature change do not completely match in the vicinity of both paths, so the optical length between the paths changes with time, which is unnecessary. May be observed as a negative signal. By using a Fizeau interferometer, such unnecessary signals can be suppressed.

In the optical microphone 214, the Fizeau interferometer 101 includes a semi-transparent mirror 102 and a reflecting mirror 35a. The semi-transparent mirror 102 is a first optical surface 102a of the semi-transparent mirror having an optical plane parallel to the reflecting surface of the reflecting mirror 35a, and is a good optical plane but non-parallel to the reflecting surface of the reflecting mirror 35a. And a second surface 102b that is not processed as a reflective surface. An antireflection film or the like may be provided on the second surface.

In the Fizeau interferometer 101, the reference path 11 when the focusing lens 31a is used as a reference is defined by an optical path from the focusing lens 31a to the semi-reflecting surface 102a of the semi-transmitting mirror 102 and returning to the focusing lens 31a. The probe path 12 is defined by the optical path from the focusing lens 31a to the reflection lens 35a until it returns to the focusing lens 31a.

In the case of homodyne detection, the optical path length difference between these two paths is detected by the photodetector 7 as a change in light intensity. Therefore, the common optical path of the reference path 11 and the probe path 12 may be short. Further, since the optical path length difference is generated in the propagation medium portion 4 in the probe path 12, the length of the optical path in other portions may be short. Therefore, if the focusing lens 31a, the semi-transparent mirror 102, the propagation medium unit 4, and the reflecting mirror 35a are arranged in this order and adjacent optical elements are arranged in contact with each other, the reference path 11 and The probe path 12 can be made extremely short. As a result, the optical element that constitutes the Fizeau interferometer 101 is prevented from vibrating due to external influences, or unnecessary signals from being included in monochromatic light as noise due to external environments such as air fluctuations and temperature changes. be able to.

Further, the Fizeau interferometer 101 is constituted by two bulk optical elements, but the adjustment thereof is only paralleling of the optical surfaces of the two elements. Therefore, the assembly and adjustment of the optical microphone 214 is easy, and the manufacturing cost can be reduced.

Further, by sharing the scattered light removal optical system including the focusing lenses 31a and 32a and the optical fiber 33a in the forward path and the return path, the number of optical components can be reduced without sacrificing the scattered light removal effect. Yes.

Similarly to the second embodiment, an area other than the two-beam interferometer in the optical microphone 214 may be replaced with an optical fiber element. An optical microphone 214 ′ illustrated in FIG. 15 includes a fiber light source 111, a coupler 41 a, a scattered light removal optical system 2 b, a Fizeau interferometer 101, a photodetector 7, and a signal processing unit 8.

The coupler 41a has three ports, and decomposes light incident from one of the three ports into light emitted from the other two ports.

Monochromatic light emitted from the fiber light source 111 enters the coupler 41a and is emitted to the scattered light removing optical system 2b. Further, the monochromatic light emitted from the Fizeau interferometer 101 and propagated through the scattered light removing optical system 2 b enters the coupler 41 a in the reverse direction and enters the photodetector 7. According to such a configuration, all the optical elements except the optical system inside the Fizeau interferometer 101 can be seamlessly connected only by the optical fiber, so that the adjustment cost can be further reduced. .

As described above, the microphone of the present invention has been described using the first and second embodiments and the first to fourth examples. However, using the configuration of the present invention, a scattering optical material is used instead of the nanoporous material. Thus, the present invention can be applied to a measuring instrument that measures optical characteristics such as a refractive index of a scattering optical material.

In addition, since it is possible to measure the optical distance of such a scattering material, a pressure sensor or gas concentration meter for measuring the pressure or concentration of the scattering gas, a gas concentration meter, a refractive index meter for gas and liquid The present invention can also be applied to the above. Further, it can be applied to a physical quantity measuring device based on refractive index measurement, a precision length measurement / shape sensing meter in a scattering medium, and the like.

The optical microphone of the present invention is suitably used for an optical microphone used for various applications. Further, it is also suitably used for a scattering gas pressure sensor, a gas concentration meter, a gas and liquid refractometer.

DESCRIPTION OF SYMBOLS 1 Monochromatic light source 2a, 2b Scattering light removal optical system 3 Two-beam interferometer 4 Propagation medium part 5 Sound collection part 6 Acoustic signal 7, 1707 Photodetector 8 Signal processing part 9 Received signal 10 Monochromatic light 11 Reference path 12 Probe path 13 DC cut filter 14 Amplifier

16 Output end 17a, 17b Ray splitting element 18a, 18b Ray reflecting element 31a, 31b, 32a, 32b Focusing lens 33a, 33b Optical fiber 34a, 34b Beam splitter 35a, 35b Reflector 36 Mach- Zehnder interferometer 41a, 41b Coupler 42 Light Fiber type Mach-Zehnder interferometer 61, 62 Lens 63 Pinhole 63b Shield plate 81 Aperture stop 91 Michelson interferometer 101 Fizeau interferometer 102 Semi-transparent mirror 111 Fiber light source 141 Conventional optical microphone 142 Nanoporous body 143 Base portion 144 Opening portion 145 Transparent support plate 147 Laser beam 148 Laser Doppler vibrometer 149 Acoustic signal 151 Light source for measuring cantilever deflection 152 Cantilever 153 Sample 154 155 Scattered light removing means 156 Light receiving means 161 Input surface 162, 164 Lens 163 Fourier transform surface 165 Output surface 201, 202, 211, 212, 213, 214 Optical microphone 1410 Reception mechanism 1411 Reflecting surface 1412 Density wave 1701 Polarizing beam splitter 1702, 1712 Non-polarization beam splitter 1703 Dual frequency light source 1704 Dual frequency light 1705 Phase comparison unit 1706, 1710 Polarizing plate 1708 Reference beat signal 1709 Probe beat signal 1711 Reference beat signal generation unit 1801 Horn 1802 Acoustic waveguide 1803 Acoustic lens 1804 Refractive surface 1805 Focus 1806 Non-reflective termination

Claims (11)

1. A light source that emits monochromatic light;
   The monochromatic light emitted from the light source is divided into two light beams, and the two divided light beams are propagated through two different paths, respectively, and then the two divided light beams are superimposed on each other and superimposed light A two-beam interferometer that emits light,
   A propagation medium portion made of a nanoporous material having a sound velocity smaller than that of air, wherein the acoustic signal incident from the incident portion crosses one of the two paths; A propagation medium that propagates through the nanoporous body;
   A photodetector for detecting light emitted from the two-beam interferometer;
   A first scattered light removal optical system that is provided between the two-beam interferometer and the photodetector and removes scattered light contained in the light emitted from the two-beam interferometer;
   A second scattered light removal optical system that is provided between the light source and the two-beam interferometer and removes scattered light contained in the monochromatic light emitted from the light source;
   An optical microphone comprising:
2. The optical microphone according to claim 1, wherein each of the first scattered light removal optical system and the second scattered light removal optical system includes a single mode optical fiber.
3. 3. The optical microphone according to claim 2, wherein the single mode optical fiber of the first scattered light removal optical system and the single mode optical fiber of the second scattered light removal optical system have the same optical characteristics.
4. The optical microphone according to claim 3, further comprising a sound collecting unit that is provided at an incident part of the propagation medium part and focuses the acoustic signal.
5. The optical microphone according to claim 4, wherein the nanoporous body is a silica dry gel.
6. The first scattered light removal optical system further includes at least one focusing lens;
   The optical microphone according to claim 5, wherein a focal point of the focusing lens is located on a core of an end face of the single mode optical fiber.
7. The optical microphone according to any one of claims 1 to 4, further comprising a signal processing unit that receives an output from the photodetector and generates a reception signal corresponding to the acoustic signal from the output by homodyne detection.
8. A reference beat signal generation unit and a phase comparison unit;
   The light source emits two linearly polarized lights having different frequencies,
   The reference beat signal generation unit generates a reference beat signal based on the two linearly polarized lights,
   5. The signal processing unit according to claim 1, further comprising: a signal processing unit configured to generate a reception signal corresponding to the acoustic signal by heterodyne detection using an output from the photodetector and the reference beat signal. An optical microphone as described in 1.
9. The optical microphone according to any one of claims 1 to 4, wherein the two-beam interferometer is a Mach-Zehnder interferometer.
10. The optical microphone according to any one of claims 1 to 4, wherein the two-beam interferometer is a Michelson interferometer.
11. The optical microphone according to any one of claims 1 to 4, wherein the two-beam interferometer is a Fizeau interferometer.

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